FRESNO, Calif. (KFSN) -- We've all heard the cautionary tale: jiggle the card reader at a gas pump to check for a skimmer. But Fresno Police say the devices aren't that simple anymore.

Instead of attaching fake card readers and keypads, scammers are planting their devices inside the pump, hidden from view.

That's why volunteers are regularly checking gas stations throughout Fresno and officers are going pump to pump, checking the interior of each compartment to see if it's been tampered with.

If the pumps are safe, they get a special tamper-evident sticker. If that sticker is broken or appears tampered with, officers suggest you use another pump.

Fresno Police say the program appears to be working and many of the criminals have moved on.

"I think it's a poor investment for them, the bad guys come in, install their skimmer and in a couple of days we pull it out," said Fresno Police Sgt. Brian Rogers. "We were finding about 1 a month before the program, now we are finding about 4 or 5 a month."

But while detectives are recovering skimmers quickly, it's much harder to catch the thieves. That's because they don't have to come back and physically retrieve the data, it's all done via Bluetooth.

"You can see them in their car, using a phone or laptop to download the data," said Fresno Police Detective Marty Lucero.

Fresno Police also say they're also seeing the scammers move away from the city to target outlying areas in the county.

They also say the safest solution is still to pay with cash or use newer model pumps where the keypad and card reader are separated.